PROJECT SUMMARY/ABSTRACT The overall goal of this NCI K08 career development proposal is to facilitate Dr. Erin Gillespie?s development into an independent investigator in cancer health services research focusing on strategies to enhance quality and evidence-based care in radiation oncology. This proposal focuses on shortening the course of radiation from 7-9 weeks to 3-5 weeks (called ?hypofractionation?) which is associated with equivalent cancer control, improved patient-reported outcomes, and decreased costs in patients with breast and prostate cancer. Nonetheless, adoption has been slow. Dr. Gillespie?s prior work has revealed that the individual radiation oncologist is highly influential in whether patients with breast cancer receive hypofractionation or a longer course of radiation. The overall hypothesis is that radiation oncologists are the linchpin in radiation treatment decision- making, and that implementation strategies that harness behavioral economics will be most likely to impact practice change. My primary objectives are to 1) identify and characterize positive deviant radiation oncologists (high users of hypofractionation) that will 2) elucidate implementation strategies and associate them with adoption of hypofractionation across various settings, and 3) pilot test a multi-pronged strategy that promotes use of hypofractionation in preparation for a large pragmatic multi-center controlled trial. The rationale that underlies the proposed research and training plan is that, with new knowledge about the levers that drive physician decision-making (beyond knowledge gaps), healthcare leaders and policymakers can optimally design and implement systems that evoke change. Dr. Gillespie will harness the resources and expertise at Memorial Sloan Kettering Cancer Center, where she is a faculty member in the Department of Radiation Oncology and the Center for Health Policy and Outcomes. She is also engaged in research in the community- based setting through the MSK Cancer Alliance. Her training plan has a foundation of implementation science that incorporates large dataset analysis, mixed methods, and behavioral economic theory. In Aim 1 of the proposed study, the investigators will analyze a Medicare dataset that includes claims linked to prescribing radiation oncologist and organization, and is supplemented by the AMA Masterfile. They will systematically conduct the first-ever evaluation of implementation strategies in radiation oncology using mixed methods. Lastly, in Aim 3 the investigators will develop and pilot test a multi-pronged strategy informed by Aims 1-2 and including audit and feedback. This proposal, together with the new skills Dr. Gillespie will acquire, will generate new knowledge about the impact of implementation strategies on de-implementation of long-course radiation ? key parameters necessary to design optimal strategies to encourage physicians to adopt high value radiation. This will specifically support the development of an implementation strategy that will be tested in a subsequent prospective randomized trial to assess real world efficacy.